Various techniques are available for eliminating the carrier frequency of an information carrying amplitude modulated signal. Such filters find common use in communication systems such as radio and television as well as many other systems where it is desired to reject a band of frequencies.
One type of filter involves simple R-C circuits and operational amplifiers in various forms. For example, an amplifier with resistance connected from the source to its input and a capacitor connected in parallel between the input and output of the amplifier may, for any frequency or more precisely for a band of frequencies, reduce a desired band of high frequencies. Such a circuit is called an active integrator and is used quite commonly. The attenuated band of frequencies is fixed by the selected parameters. If another band of frequencies is to be rejected, the values of the components must, of course, be changed to provide the desired response.
Another type of filter is a band reject filter which may be comprised of two integrators in tandem each acting as a delay to give phase lag and lead at frequencies determined by the parameters of the circuits, and by combining the lead and lag voltages will null out or reject a frequency or band of frequencies.
Innovations in the field of filters has also resulted in the use of digital filters wherein an input signal is sampled and delayed at rates dependent on the sampling period. The actual sampling rate in these digital filters determines the band pass or passband reject band of frequencies. In such filters sampling the input signal provides the delay just as the integrator circuits provide lag in the first two types of filters discussed above. Such filters are discussed in a textbook entitled "Digital Filters and Their Applications" by V. Cappellini, et al., Academic Press, 1978.
Thus, if a particular incoming signal has a carrier whose frequency varies due to some phenomenon, the frequency which it is desired to reject will change. Since the above discussed filters are tuned for one frequency, they are inherently not capable of rejecting the frequency caused by variation in the frequency of the input signal.
The present invention overcomes this problem.